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Thread: FANS

  1. #1 FANS 
    ddd is offline
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    Aug 2010

    I'm currently conducting investigation on how the surface area (or width) of a blade on a 3-bladed fan effect the power of the wind produced. I have done lots of research and it seems that there has been lots of experiments and calculations done on the effects of the number of blades and the effects of radius/diameter on power. However, there is little information on the effects of the surface area.

    I was wondering whether someone can help me explain the relationship between the surface area of the blades and the power, whilst keeping the radius and number of blades fixed?

    I have collected some data through an experiment where I had fans of the same radius but different surface area. I measured the velocity of the wind for each of the different fans.

    To process my data, I decided to calculate the power, using the equation:
    Power = 0.5*Area*Density of air*(velocity of wind)^3

    I got this equation from my textbook regarding the calculation of maximum available power of wind energy (for turbines). And from research, I have found out that the Area is equal to swept area or the area of the full circle covered by the fan.

    The question and problem I am facing is that if I use the area of the circle (swept area), my results will not have much difference as the area of the circle (swept area) is equal for each fan (and the velocities didn't have that much of a difference).

    So if I want to show a difference in my result, is it better to use the total surface area of the blades instead of the area of the full circle? I mean, at any one point in time, the area of air that is getting pushed by the fan is where the blades are... - so wouldn't the total surface area of the blade prove to be a more accurate measure of the power produced by the fan?

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  3. #2  
    Forum Professor
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    Apr 2007
    In the circuitous haze of my mind
    The total surface area of the fan blades would determine (in a sense) how often per second the system pushes the air. A smaller surface area should result in a higher RPM till equilibrium (the point where the energy put in equals the energy given out, in other words the result of the fan's spin up). The higher RPM would then allow for higher pressure (velocity) exhaust air.

    Maintaining the same surface area, but simply varying how it is distributed (many blades each with a smaller surface area, or fewer blades each with a larger surface area) will alter how the atmosphere reacts with your fan system. Fewer, larger blades should result in larger lower pressure regions, but less often.....more, smaller blades would result in smaller low pressure regions more often.

    There are various trade offs when you change the design. The smaller blade design for example would strain each blade less, but create a tighter low pressure area due to the decreased space between the top of the previous blade, and the bottom of the next blade. This could potentially decrease the efficiency of the system by making it more difficult for the air to fill in the voids created by the blades. This also means though that larger blades would create larger, more turbulent regions of air in their wake.

    Ahh, this is just speculation, so feel free to correct me if I am wrong.

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  4. #3  
    Forum Isotope Bunbury's Avatar
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    Sep 2007
    explain the relationship between the surface area of the blades and the power, whilst keeping the radius and number of blades fixed
    Presumably you're referring to an axial fan. It's not a simple relationship because there are other variables, such as how the area of the blade is disposed, and its angle of attack. The part of the blade near the hub moves with a lower velocity than the part near the tip so most manufactureres will put more area and a greater angle of attack near the hub, resulting in some curvaceous designs. Many ceiling fans, on the other hand, have simple, straight and highly inefficient blades.

    Most of the fan manufactureres publish fan rating data, but it tends to be empirical and specific to their products and you can't get at the fundamental physics. Look at sections 6.8 and 6.9 of this publication for instance.
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